This invention in general relates to aerial lift apparatus, and in particular relates to apparatus for moving a structure such as an operator's platform, basket, tools or material to an elevated position.
A number of different designs have been provided in the prior art for raising a platform or other structure to an elevated position to perform various tasks, such as in the construction industry or for maintenance work to move workers, tools or materials to different elevations.
For reaching higher elevations the prior art apparatus have employed multiple booms in which a lower boom is raised relative to a support frame, and one or more secondary booms are raised relative to the lower boom. In many instances it is necessary that the platform maintain a level attitude as it is raised. To accomplish this the prior art apparatus have employed different design solutions. However, these prior art apparatus have not been completely satisfactory because of their complexity, cost of construction, high maintenance requirements, and the necessity for periodic field adjustment to maintain proper operation. Although certain prior art devices operate with manual sychronization of the lower and secondary booms, it is desirable to provide automatic machine controlled synchronization.
Among the prior art devices of the foregoing nature are those in which the lower boom is raised from the support frame by means of a first hydraulic cylinder while the secondary boom is raised relative to the lower boom by means of a second hydraulic cylinder, with hydraulic circuitry being provided to extend and retract the cylinders conjointly so that the two booms are moved in a manner which maintains a level orientation of the platform. However, in such an arrangement it is difficult to maintain the booms in proper synchronization because of factors such as fluid leakage from the cylinders and changes in oil viscosity with temperature. Moreover, such a design is relatively complex and expensive to build and maintain.
Another prior art elevating platform apparatus is of the type shown in U.S. Pat. No. 3,231,044 which provides a pair of articulated tubular booms. The upper boom is elevated relative to the lower boom by means of a cable which is reeved around a pulley secured to the side of an hydraulic actuator mounted within the lower boom. With the actuator hydraulically locked, the relative movement between the actuator and the lower boom as the latter is elevated causes the cable to pivot the upper boom. Such an apparatus, however, is relatively complicated and expensive, and requires periodic maintenance and adjustment of the cable and pulley arrangement. Thus, there is a requirement for an aerial lift apparatus which will obviate the problems and shortcomings of existing designs.